Electrically regeneratable filter element

ABSTRACT

An electrically regeneratable filter element comprises at least two flanks, each of these flanks comprising a stiff material layer. Each of these flanks has at least one thermally and electrically insulated side. The filter element comprises further a metal fiber fleece being pleated according to pleating lines providing an edge with pleat openings. The metal fiber fleece is mounted between the flanks, in such a way that the thermally and electrically insulated sides make contact with the edge, meanwhile these sides closing the pleat openings.

FIELD OF THE INVENTION

[0001] The present invention relates to filter elements, which may beregenerated electrically. More specific, the invention relates to filterelements for filtering diesel exhaust gasses.

BACKGROUND OF THE INVENTION

[0002] Diesel soot particulate traps comprising pleated metal fiberfleece are, known, e.g. from U.S. Pat. No. 5,709,722.

[0003] Diesel soot particulate traps, which can be regenerated viaelectrical heating of the filter element itself, are known, e.g. fromU.S. Pat. No. 5,800,790.

[0004] The presently known filter elements, suitable for electricalregeneration, have the disadvantage that most of the thermal energy,obtained by Joule effects out of electrical energy and used to heat thefilter element, is lost due to thermal losses.

SUMMARY OF THE INVENTION

[0005] It was found that the losses of thermal energy is caused by 3effects:

[0006] 1. The filter medium, generating the thermal energy via Jouleeffects, looses thermal energy via radiation, e.g. towards the filterhousing.

[0007] 2. Thermal energy is lost via convection, heating the gasseswhich pas through the filter medium during regeneration. This effect ismuch larger when the strip is regenerated in stream.

[0008] 3. Thermal energy is lost due to thermal conduction. E.g. whenthe filter medium is welded to the housing, a lot of thermal energy istransferred from the filter medium to the housing via this contact. Thehousing is needlessly heated by this thermal energy conducting.

[0009] It is an object of the invention to provide a filter element, tobe regenerated electrically, which has a reduced thermal energy loss.Further, it is an object of the present invention to improve the contactbetween filter medium, being electrically regeneratable, and the housingof the filter element.

[0010] It is also an object of the invention to provide a filter unit,comprising at least two but possibly more than two filter elements, eachfilter element being regeneratable individually. Such a filter unit assubject of the invention may be used in a diesel exhaust filter pack forstationary diesel engines of for diesel engines, used in vehicles suchas boats, trains or other motor vehicle.

[0011] Filter pack is to be understood as a filter system which isinstalled or used in a gas stream. It comprises a gas inlet, a gasoutlet, and at least one filter unit, installed between inlet andoutlet.

[0012] A filter element as subject of the invention comprises a pleatedmetal fiber fleece. This metal fiber fleece, preferably sintered, ispleated according to pleating lines, so providing a edge with pleatopenings. The gas, to be filtered, has to flow from one side of thefleece (inflow side) to the other side of the fleece (outflow side),passing through the fleece. Appropriate pleat openings have to be closedin order to make the gas to flow through the metal fiber fleece, sopreventing bypasses from gas from the inflow side to the outflow side,without passing through the metal fiber fleece.

[0013] A filter element according to the invention further comprises afilter element housing, which comprises at least two flanks, eachcomprising a stiff material layer. According to the present invention,at least one side of each flank is provided with thermally andelectrically insulating properties, hereafter referred to as “thermallyand electrically insulated side”. “Stiff material” is to be understoodas an inflexible material, which to a certain extend lacks suppleness orpliability and having the property of being difficult to bend, as isgenerally known for ceramic or metal plates.

[0014] According to the invention, the edge of the pleated metal fiberfleece is mounted between the two thermally and electrically insulatedsides of the flanks in such a way that the edge makes contact with thesethermally and electrically insulated sides of the flanks. The flanksexercise a clamping force on the edge of the metal fiber fleece in adirection essentially parallel to the pleating lines, meanwhile closingthe pleat openings in order to prevent bypasses.

[0015] Preferably, the metal fiber fleece and the flanks provide ahollow filter volume. At least one, but preferably two flanks areprovided with an aperture, which provides entrance to gasses to theinner side of the hollow filter volume. Gas, filtered or to be filteredmay flow in or out of the hollow filter volume via these apertures.

[0016] An improved connection may be obtained is many different waysaccording to the invention.

[0017] Each flank may comprise a ceramic plate, being the stiff materiallayer, which comes into contact with the edge. The metal fiber fleece isclamped between those two ceramic plates. Both flanks exercise aclamping force on the edge of the metal fiber fleece in a directionessentially parallel to the pleating lines, meanwhile closing the thepleat openings in order to prevent bypasses. The ceramic plate providesthermal and electrical insulating properties to the flanks. Thethickness of the ceramic plates is preferably at least 5 mm, mostpreferably at least 6 mm, e.g. at least 10 mm.

[0018] Preferably these ceramic plates are provided with recesses. Thedepth of these recesses is preferably larger than 0.5 mm, and may be inthe range of 0.5 mm to 2 mm, e.g. 1.58 mm. These recesses are obtainableby providing e.g. a slot in the thermally and electrically insulatingceramic plates. These recesses correspond with the edge, in such a waythat they engage closely with the edge when the pleated metal fiberfleece is mounted between the two flanks. The edge of the metal fiberfleece is sunken over a certain depth in the recesses. The part of theedge of the metal fiber fleece, sunken in the recesses is hereafterreferred to as “sunken part”.

[0019] It should be noted that the edge is installed in the recesses insuch a way that small movements, e.g. thermal expansions or vibrations,of the pleated metal fiber fleece can be allowed. This freedom ofmovement is obtained by providing recesses, which are slightly deeperthan the height of the sunken part of the edge in the thermally andelectrically insulating side

[0020] The ceramic plates are provided by using ceramic materials, e.g.based on Al₂O₃ and or SiO₂ or mica. The flank may be provided out of onematerial, or may comprise different layers, provided by differentmaterials. One understands that, in case of recesses used and in casedifferent layers are used to provide the flanks, the recesses are to beprovided in layers, which are thermally and electrically insulating. Toprotect the ceramic plates against mechanical damages, the ceramicplates may be supported by a metal plate, being present at the otherside of the ceramic plate, not contacting the metal fiber fleece.Alternatively, this metal plate may have the shape of a rim, in whichthe ceramic plate fits.

[0021] Less preferably, although possibly according to the presentinvention, the metal fiber fleece is glued to the ceramic plate usingceramic or high temperature resistant adhesive.

[0022] Alternatively, each of these flanks comprising a thermally andelectrically insulating fabric and a stiff material layer. The thermallyand electrically insulating fabric is present at one side of the stiffmaterial layer, so providing a thermally and electrically insulated sideto the flank. The metal fiber fleece is mounted between the thermallyand electrically insulated sides of both flanks, which exercise aclamping force on the edges of the metal fiber fleece in a directionessentially parallel to the pleating lines, meanwhile closing the pleatopenings in order to prevent bypasses.

[0023] For each flank, a thermally and electrically insulating fabric,e.g. a ceramic textile layer, is supported by a stiff material layer,preferably a metal or ceramic plate or rim.

[0024] The metal fiber fleece is mounted between the thermally andelectrically insulated sides of both flanks, in such a way that thesesides of the flanks close the pleating openings. Since the thermally andelectrically insulating fabric provides the thermally and electricallyinsulated side, the thermally and electrically insulating fabrics makecontact with the edge of the metal fiber fleece. A clamping force isexercised by the flanks on the edges of the metal fiber fleece in adirection essentially parallel to the pleating lines. Since the pleatedmetal fiber fleece has sufficient buckling resistance, the pleated metalfiber fleece is pressed into the thermally and electrically insulatingfabric, so providing a recess over the edge of the pleated metal fiberfleece in the thermally and electrically insulating fabrics of theflanks.

[0025] The depth of the recess of the edge should at least be sufficientto prevent the pleated metal fiber fleece to move along with the gas tobe filtered. This phenomenon is so called ‘blow through’. The recess,being the depth over which the metal fiber fleece is presses in thethermally and electrically insulating fabric, is preferably larger than0.5 mm, but may be in the range of 0.5 mm to 2 mm.

[0026] In the scope of the present invention, a thermally andelectrically insulating fabric is to be understood as a nonwoven, woven,braided or knitted textile fabric, comprise thermally and electricallyinsulating fibers at the surface of the fabric, which is to contact theedge of the metal fiber fleece. Most preferably, the whole fabricconsist of such thermally and electrically insulating fibers, however, acombination of thermally and electrically insulating fibers at the sidecontacting the edge, with metal fibers at the opposite side may be used.Such thermally and electrically insulating fibers preferably are ceramicfibers, such as fibers, comprising Al₂O₃ and/or SiO₂, e.g.NEXTEL®-fibers.

[0027] The fabric thickness is preferably between 3 to 6 mm. A woven ornonwoven fabric is preferred.

[0028] When flanks comprising ceramic plates or rims, together with athermally and electrically insulating textile fabric, are used toprovide a filter element as subject of the invention, the ceramic platesor rims are obtainable by using ceramic materials, e.g. based on Al₂O₃and or SiO₂ or mica to provide this side of the flank. When flankscomprise metal plates or rims, together with a thermally andelectrically insulating textile fabric, preferably stainless steel isused to provide the metal plates or rims. The flank may be provided outof one material, or may comprise different layers, provided by differentmaterials.

[0029] Another filter element as subject of the invention may beprovided using a relatively thick layer of ceramic adhesive to connectthe metal fiber fleece and a stiff material layer, preferably a metalplate or rim to each other. The adhesive is at least present over thewhole length of the edge of the metal fiber fleece, but preferably, thewhole surface of the side of the stiff material layer is coated withthis adhesive. The metal fiber fleece is mounted between the flankshaving its pleating lines preferably essentially perpendicular to thestiff material layer of the flanks. The layer of ceramic adhesive is toprevent direct contact over the total length of the edge of the metalfiber fleece, being connected to the flank. It positions the metal fiberfleece, provides the electrically and thermally insulating propertiesand offers a good seal between the metal fiber fleece and the stiffmaterial layer. A thermally and electrically insulating side is soprovided to the stiff material layer. The metal fiber fleece is mountedbetween the thermally and electrically insulated sides of both flanksprovided by the adhesive. The flanks exercise a clamping force on theedges of the metal fiber fleece in a direction essentially parallel tothe pleating lines. Meanwhile these flanks close the pleat openings inorder to prevent bypasses.

[0030] In order to improve the adhesion between stiff material layer andceramic adhesive, a wire mesh, an expanded or perforated metal sheet maybe inserted between the surface of the stiff material layer and the edgeof the metal fiber fleece. This mesh or expanded or perforated metalsheet acts so to say as anchoring points for the ceramic adhesive, andit is sunken in the adhesive layer. Best results were obtained using ametal rim and a metal mesh. The metal mesh was spot welded to the metalrim on several points.

[0031] The thickness of the adhesive layer is preferably more than 0.5mm, and less than 2 mm. The edge of the metal fiber fleece is sunkenover a certain depth in the adhesive layer, providing a so-called sunkenpart to the edge of the metal fiber fleece. This sunken part has aheight of preferably at least 10% less than the thickness of theadhesive layer, but also preferably in the range of 0.5 mm to 2 mm.

[0032] Further, the adhesion between stiff material layer and ceramicadhesive may be obtained by first coating, e.g. spraying a layer ofceramic particles (e.g. by flame spraying of Al₂O₃ or SiO₂) on the sideof the stiff material layer, before providing the ceramic adhesive tothe stiff material layer. This layer also further improves theelectrical insulation between metal fiber fleece and stiff materiallayer, which may be required in case the stiff material layer is a metalplate or rim. Such spraying may be done also on the mesh or perforatedor expanded metal sheet. Possibly a ceramic layer is sprayed on the meshor expanded or perforated sheet, after it has been spot welded to e.g. ametal rim or plate.

[0033] To further improve the ductility and the resistance to thermalcycling of the ceramic adhesive layer between the stiff material layerof the flanks, being a ceramic or a metal plate or rim, and sinteredmetal fiber layer, metal particles may be added to the ceramic adhesive.Metal short fibers are preferred over metal powder, since the ductilityof cured ceramic adhesive is much more superior as compared to ceramicadhesive comprising metal powder. Surprisingly it was found that theelectrical insulation properties of such adhesive layer were influencedonly slightly, as compared to pure ceramic adhesion.

[0034] Short metal fibers preferably comprises fibers with an equivalentdiameter “D” between 1 and 150 μm preferably between 2 and 100. Mostpreferably the diameter ranges between 2 and 50 μm or even between 2 and35 μm such as 2, 4, 6.5, 8, 12 or 22 μm. Preferably, but notnecessarily, short metal fibers have an L/D-ratio of more than 5,preferably more than 10, wherein L stands for the average length of theshort metal fibers.

[0035] Preferably, the layer of ceramic adhesive comprises at least 0.5%by weight of short metal fibers, most preferably more than 10% by weightor even more than 20% by weight. Preferably the layer of cerainicadhesive comprises less than 30% by weight of short metal fibers.

[0036] A metal plate or rim is preferably provided out of stainlesssteel. Most preferably the metal fiber of the metal fiber fleece and themetal plate or rim are out of the same metal alloy.

[0037] Filter elements, as subject of the invention may further compriseother elements, to form, together with the flanks mentioned above, thefilter element housing. These elements may also be thermally andelectrically insulated, in order to reduce the thermal energy, lost dueto radiation, from the metal fiber fleece to these elements or due tothe heating of these elements because of contact between hot gas andhousing. E.g. a perforated metal screen or a more permeable thermallyinsulating fabric may be applied, in order to further reduce the thermallosses due to radiation towards the adjacent filter units of the filterpack wall. In case of a more permeable thermally insulating fabric,preferably, a SiO₂-grid woven fabric is used.

[0038] Such filter elements as subject of the invention have severaladvantages.

[0039] The thermal energy loss due to conduction is prevented, since thesides of the flanks, used to close the pleat openings have thermallyinsulating properties. The metal fiber fleece is only in contact withthe filter housing via this side. The pleating of the metal fiber fleecealso causes thermal radiation, being radiated from one pleat to theadjacent pleats. Since electrical current is to be supplied only to themetal fiber fleece, in order to regenerate the fleece, the fleece iselectrically insulated from the filter housing at its edge, by theelectrically insulating side.

[0040] Preferably, the metal fiber fleece is to be resistant to bulging.A sintered and pleated metal fiber fleece has a rather high bulgingresistance due to the pleated shape, to provide a edge.

[0041] Further, surprisingly it was found that, when a filter element assubject of the invention comprising a thermally and electricallyinsulating fabric is used, e.g. to filter diesel exhaust gas, loadedwith soot particles, the filter element works self-sealing, even afterregenerating. This is explained as follows.

[0042] The edge of the metal fiber fleece is mounted or pressed betweenthe thermally and electrically insulating sides of the flanks.

[0043] In case a thermally and electrically insulating fabric is used,due to the textile nature of the fabric, the metal fiber fleece isrecessed to a certain depth in the fabric. Under normal circumstances,this recess is sufficient to close all voids in the fabric next to therecessed part of the metal fiber fleece, so no gas can bypass the metalfiber fleece through the thermal and electrical insulating fabric. Incase there is a small void in the fabric, which is not closed by therecessed part of the metal fiber fleece, small amounts of exhaust gaswill bypass the metal fiber fleece via this void. The soot, beingpresent in the exhaust gas, will be trapped by the fabric, so closingthe void space. When the metal fiber fleece is now regenerated, thethermal and electrical insulating fabric will not be heated enough inorder to incinerate the soot, trapped by the fabric at the void space.So the bypass of gas through the fabric is hindered after the voidspaces are filled with soot, due to such bypass. The filter sealsitself.

[0044] An identical effect is obtained when the edge of the pleatedmetal fiber fleece is mounted in a recess in the thermally andelectrically insulating side of a flank, being the ceramic plate.Preferably a small void space is provided underneath the edge, to allowsmall movements. The recess fits that good to the sunken part of edge atthe surface to the pleated metal fiber fleece, that under normalcircumstances, no gas can bypass the metal fiber fleece via the sides ofthe edge and these voids. In case there is a small gap between the sideof the edge at a sunken part and the slot, soot will be trapped andretained in these gaps. When the filter is regenerated, the soot willnot be heated enough in order to incinerate this soot completely. So thebypass of gas through the gaps is hindered after the gaps are filledwith soot, due to such bypass. The filter seals itself.

[0045] In the scope of the present invention, with metal fiber fleece ismeant a fleece, comprising metal fibers, preferably steel fibers. Thealloy of metal or steel may be chosen dependant on the temperature rangewhich is to be withstand by the metal fiber fleece. Stainless steelfibers of AISI alloys of the 300- or 400 series, or alloys such asInconel® are to be preferred. In case high temperatures are to bewithstand during regeneration, alloys comprising Fe, Al and Cr arepreferred, such as Fecralloy®. The fibers may be obtained by anypresently known production method, such as bundle drawing or shaving.Fiber diameters between 1 and 100 μm are to be used, preferably between2 and 50 μm, e.g. between 12 and 35 μm such as 12, 17 and 22 μm.preferably the fleece is sintered using appropriate sinteringcircumstances, according to the alloy used. Preferably, the metal fibersare obtainable by bundle drawing or coil shaving. The latter isdescribed more in detail in WO97/04152.

[0046] Also thickness, weight per m², pore diameter and other fleeceparameters may be chosen, according to the particles which are to beretained and/or the application for which the filter element is to beused.

[0047] Preferably, the metal fiber fleece used to provide the filterelements as subject of the invention, comprises different layers ofmetal fibers. Each fiber layer comprises fibers with a certainequivalent diameter. Best filtering results were obtained when a layerwith the coarsest fibers is facing the inflow side of the filterelement, whereas a layer of metal fibers with the finest fibers isfacing the out-flow side of the filter. An example of such layered metalfiber fleece is a metal fiber fleece comprising a layer of metal fiberswith equivalent diameter of 35 μm, and a layer of metal fibers with anequivalent diameter of 17 μm. Possibly a layer of metal fibers withequivalent diameter of 22 μm can be located between these two layers.Porosity of more than 85% is preferred, while the weight per squaremeter of the fleece is preferably less than 1500 g/m², e.g. 1450 g/m².

[0048] Equivalent diameter is to be understood as the diameter of aradial cut of an imaginary round fiber, having an identical surface asthe radial cut of the fiber under consideration.

[0049] According to the present invention, preferably the metal fiberfleece consists of only one strip of filter media comprising metalfibers. Most preferably, this strip is rectangular. Howeveralternatively, the metal fiber fleece may consist of more than one stripof filter media comprising metal fibers which strips are mounted betweenthe two flanks of the filter element as subject of the invention.

[0050] Sintered metal fiber fleece has a good resistance againstbuckling, when put under mechanical load in a direction, parallel to theplane surface of the fleece. To improve the buckling resistance, thefleece may be corrugated using preferably repetitive undulations, with awavelength preferably less than 5 times the thickness of the fleece. Theamplitude of the corrugation is also preferably less than 5 times thethickness of the fleece. The buckling resistance may be improved morethan 50% in ambient circumstances. Then the fleece is heated to morethan 600° C., the buckling Improvement is still more than 30%.

[0051] The metal fiber fleece, used to provide a filter element assubject of the invention further comprise at least two but possibly morethan two contact bodies, fixed, e.g. clamped on or sintered to the metalfiber fleece. According to the present invention, a contact body is abody to which the electric current is supplied by the electric circuit,in order to regenerate the filter element. This contact body divides ina proper way the electric current over the total surface of the metalfiber fleece. Preferably, these contact bodies are metal foils, e.g.Ni-foil or metal woven meshes, sintered at both ends of the metal fiberfleece.

[0052] Special care is to be taken in case the metal fiber fleece ispleated in such a way that both ends of the metal fiber fleece, each ofthem to be contacting one pole of the electric circuit, are locatedclose to each other. The contact bodies are to be insulated from eachother. This can be done by inserting one or more electrically insulatingplates between both contact bodies, e.g. mica plates. Both contactbodies may be connected to this electrically insulating plate usingbolts and nuts or alike. Preferably, the contact bodies are applied onthe ends in such a way that the contact bodies extend from the metalfiber fleece in the off-stream direction of the filter element.

[0053] Filter elements as subjects of the invention are used to providefilter units. Several filter elements may be combined, e.g. stacked oneon top of the other. To avoid thermal losses, the different filterelements are separated from each other by a thermally insulating layer,e.g. a thermally insulating and thermal resistant layer of textile, e.g.a woven SiO₂-fabric around the filter unit, a more permeable thermallyinsulating fabric may be applied, in order to further reduce the thermallosses due to radiation towards the adjacent filter units of the filterpack wall. Preferably, a SiO₂-grid woven fabric is used.

[0054] Filter elements as subject of the invention may be used to filterhot gases, such an exhaust gases from diesel internal combustionengines.

[0055] As subject of the invention a filter unit comprising filterelements is provided using filter elements with two flanks, each flankhaving an aperture which provides entrance for gasses to the inner sideof the hollow filter volume. The filter elements are mounted over apermeable core member, e.g. a perforated metal tube, which extendsthrough the apertures of the filter elements. Gas which is provided tothe permeable core member, may enter in the inner side of the hollowfilter volume. Such gas are to flow through the metal fiber fleece ofthe filter elements towards the outside of the filter elements.Alternatively, gas, flowing through the metal fiber fleece entering inthe inner side of the hollow filter volume, may be evacuated via thepermeable core member.

[0056] Several filter elements or filter units comprising filterelements as subject of the invention may be used in parallel, e.g. to beable to regenerate at least one filter element, through which no gasflows, so reducing convection heat losses, while the other filterelements continue to filter the gas stream. They may be mounted inseries connection, to filter the gas stream in different steps, e.g. fordifferent particle sizes.

[0057] Further, as another subject of the present invention, a filtersystem is provided comprising several filter units as subject of theinvention. A filter system further comprises a valve system and anelectrical control system. Temporarily, one or more filter units may beshut from the gas stream to be filtered. Hereafter referred to as “putoff-line”. This is done by closing and opening appropriate valves of thevalve system. The closing of the valves is controlled by the electricalcontrol system. The electric control system further controls theprovision of electrical current to one or more filter elements, andpossibly the temperature and pressure in the filter system. Also, ifnecessary, the timing of the regeneration sequences of the differentfilter units may be controlled by this electric control system.

[0058] So, each filter element can be regenerated individually,preferably one after the other. Alternatively, the filter element may beregenerated inline, while gas continues to flow through the filterelement.

[0059] Preferably, at least one and most preferably all flanks have anaperture, e.g. a circular opening in the middle of the flank. Suchapertures may be used to mount several filter elements as subject of theinvention one on top of the other.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The invention will now be described into more detail withreference to the accompanying drawings wherein

[0061]FIG. 1 shows schematically a general view of a filter unit assubject of the invention

[0062]FIG. 2 shows schematically an enlarged view of part AA′ of thefilter unit of FIG. 1.

[0063]FIG. 3 shows schematically a section according to the plane BB′ ofthe filter unit of FIG. 1.

[0064]FIG. 4, FIG. 5a and FIG. 5c show a detail AA′ of an alternativefilter element as subject of the invention;

[0065]FIG. 5b shows a section according to CC′ of the filter element ofFIG. 5a;

[0066]FIG. 5d shows a section according to CC′ of the filter element ofFIG. 5c;

[0067]FIG. 6 shows schematically a side view of the contact bodies froma filter element as subject of the invention.

[0068]FIG. 7 shows schematically a view of alternative contact bodiesfrom a filter element as subject of the invention.

[0069]FIG. 8, FIG. 9 and FIG. 10 show schematically a section accordingto the plane BB′ of an alternative embodiment of a filter unit assubject of the invention.

[0070]FIG. 11 shows a diesel exhaust filter system in a muffler-likeshape, comprising different filter units as subject of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0071] A preferred filter unit as subject of the invention is shown inFIGS. 1, 2 and 3.

[0072] The filter unit comprises a number of filter elements 11, whichare stacked one on top of the other. They all have a ring-like shape. Aperforated metal tube 12 is positioned inside the inner opening 13 ofthe filter element. Between each filter element, a disc-like SiO₂ feltmaterial 14 is positioned to thermally insulate the different filterelements from each other. At both ends of the filter unit, a metal plate15 is fixed against the upper and lower filter element e.g. as shown inFIG. 1 by means of a screw 16, which pushes the plate towards the filterelement. Between this plate 15 and the upper or lower filter element,another disc-like SiO₂ felt material 14 is positioned.

[0073] When this filter unit is used, preferably the gas to be filteredflows in from the outer side of the filter elements (indicated witharrow 17), through the filter medium 18 through the perforations of themetal tube 12, to the further exhaust system as indicated with arrow 19.

[0074] Taking each filter element of the present embodiment intoconsideration, a metal fiber fleece is used as filter medium 18. The‘dirty’ gas flows in via the inflow side 20, through the metal fiberfleece, via the outflow side 21 of the metal fiber fleece to the exhaustsystem. The metal fiber fleece is connected via two contact bodies 22and 23 to an electric circuit 24, providing electrical current to themetal fiber fleece in order to regenerate the dirt, e.g. soot, trappedin and on the filter medium. The metal fiber fleece is preferablypleated in such a way that the thermal radiation heat, generated by thepleats 25 during regeneration, radiates to the adjacent pleats, asindicated by arrows 26. An important reduction of electrical power isobtained using this radiation heat to propagate and support thecombustion of the filtered particles

[0075] The set-up of a preferred embodiment of the filter element isshown in FIG. 2. A flank 28 of the filter element comprises a metal rim29, to which a wire mesh 30 is spot welded on several spots 31. A finelayer of ceramic material Al2O3 32 was sprayed on the electrical andthermal insulating side 33 of the flank. A relatively thick layer ofceramic adhesive 34 was applied on this mesh and the electrical andthermal insulating side 33, before the metal fiber fleece 18 was adheredto this ceramic adhesive 34, which comprises more than 10% of weight ofshort metal fibers.

[0076] The thickness of the adhesive layer was 2 mm and an adhesivebased on ZrO2-MgO compound was used. The edge of the metal fiber fleecewas sunken in the adhesive layer over a depth of 1.5 mm.

[0077] Metal plate and metal mesh were provided out of stainless steelAISI 304. Alternatively stainless steel AISI 430 was used.

[0078] The set-up of a detail AA′ of an alternative embodiment of thefilter element is shown in FIG. 4. A flank 401 of the filter elementcomprises a stiff material layer 402, being a metal rim, in which anelectrically and thermally insulating fabric 403 is located. This fabric403 preferably is a SiO₂-feltlike material (e.g. non-woven), having athickness of approximately 3 mm. The pleated edge of the metal fiberfleece 18 is squeezed between two electrically and thermally insulatingsided of the flanks.

[0079] When mounted, the metal fiber fleece 18 is pressed in the fabric403 over a depth 404 of approximately 1 mm. This recess avoids theblow-through of the metal fiber fleece once the filter element is inuse.

[0080] The set-up of a detail AA′ of alternative embodiments of thefilter element is shown in FIGS. 5a and 5 c.

[0081] A detail AA′ of a fist alternative embodiment is shown in FIG.5a. A section according to the plane CC′ of this embodiment is shownschematically in FIG. 5b. A flank 501 of the filter element comprises ametal rim 502, in which a ceramic plate 503 is provided. This ceramicplate is based on Al₂O₃-ceramic material or SiO₂-material and has athickness of approximately 6 mm. The ceramic plate 503 is provided witha recess 504 having a depth 505 of 2 mm. The edge of metal fiber fleece18 is sunken into the recess 504, so providing a sunken part 506 to theedge of metal fiber fleece 18 having a height 507 of approximately 1.5mm.

[0082] A detail AA′ of a second alternative embodiment is shown in FIG.5c.

[0083] A section according to the plane CC′ of this embodiment is shownschematically in FIG. 5d.

[0084] A flank 510 of the filter element comprises a metal rim 511, inwhich a ceramic plate 512 is provided. This ceramic plate is based onAl2O3-ceramic material or SiO₂-material and has a thickness ofapproximately 6 mm. At the inner side of the ceramic plate 512, which isto make contact with the metal fiber fleece 18, ceramic glue 513 isprovided. The edge of metal fiber fleece 18 is sunken into the glue 513.This relatively thick layer of ceramic adhesive 513 based on ZrO₂-MgOcompound, comprises more than 10% of weight of short metal fibers,preferably being stainless steel fibers having an equivalent diameter of22 μm.

[0085] To improve the resistance to the mechanical tension, due to thefixation of the different elements on top of each other by screw 16,several studs 35 may be welded to the upper and lower rim of each filterelement. As shown in FIG. 1, FIG. 2, FIG. 4, FIG. 5a, FIG. 5b, FIG. 5cand FIG. 5d, around the filter element 11, a perforated metal plate 39may be present (as only shown partially in the Figures for the sake ofclarity).

[0086] Turning now to the contact bodies 22 and 23 of the preferredembodiment as shown in FIG. 6 and FIG. 7, a fine Ni-sheet 36 wassintered to the ends of the metal fiber fleece. Both contact bodies werebrought together and fixed to an insulating plate 37, e.g. a mica-plateby means of two bolts 38 and 39. In order to avoid electrical contactbetween contact body 22 and bolt 38, and between contact body 23 andbolt 39, two mica sheets 40 were inserted between the insulating plate37 and the contact bodies 22 and 23.

[0087] An alternative set-up is shown in FIG. 7. An identical set-up asin FIG. 6 is used, but the contact body 22 is shaped in such a way thatno material of this contact body 22 is present at behind bolt 38, fixingthe contact body 23 to the insulating plate 37. Identically, the contactbody 23 is shaped in such a way that no material of this contact body 23is present at behind bolt 39, fixing the contact body 22 to theinsulating plate 37. Using such contact bodies, the use of two micaplates 40 may be avoided, which may simplify the construction of thefilter element.

[0088] An alternative cut according to BB′ is shown in FIG. 8. Theperforated tube in this embodiment has an elliptic section. Also here,the metal fiber fleece is pleated according to pleating lines, whichenables radiation from one pleat to another during regeneration.

[0089] Another alternative cross section of a filter element as subjectof the invention is shown in FIG. 9. The filter element in thisembodiment comprises two metal fiber fleece strips, which together formthe whole filter media of the filter element. Both metal fiber fleecestrips have two contact bodies (22 and 23), at one end each, which areconnected to an appropriate electric circuit 24.

[0090] Another alternative cross section of a filter element as subjectof the invention is shown in FIG. 10. The filter element comprises a setof metal fiber fleece strips, each being pleated over one pleating line81. All strips are mounted side by side. Each metal fiber fleece striphas two contact bodies (22 and 23), one at each end of the strip. Thecontact bodies are lined up and connected to an appropriate electriccircuit 24.

[0091] As shown in FIG. 11, gas to be filtered may enter into a mufflersystem, via inlet 91. Several filter units 92, each comprising severalfilter elements 93 are present in the muffler-like system. The gas to befiltered goes, as indicated with arrow 94, through the filter media ofeach filter element and leaves the filter unit 92 via the perforatedtube 95 in a collecting chamber 96. Via an outlet 97, the filteredexhaust gas flows further through the exhaust system as indicated witharrow 98.

[0092] As filter medium, a sintered metal fiber fleece comprising threelayers of stainless steel fibers is used. A first layer comprises 600g/m² of Fecralloy® fibers with equivalent diameter of 17 μm. A secondlayer of Fecralloy® fibers is applied on top of the first layer. Thislayer comprises 250 g/m² of fibers with equivalent diameter of 22 μm. Athird layer of Fecralloy® fibers is applied on top of the second layer,having fibers with equivalent diameter of 35 μm. This third layercomprises 600 g/m² fibers.

[0093] A soot retention of 91% was obtained, using a stainless steelfleece, having a porosity of 85%.

[0094] The length of the metal fiber fleece in the above describedembodiments is preferably 1200 mm, while the hight of the metal fiberfleece strip is preferably between 30 and 35 mm, e.g. 33.75 mm.

[0095] The soot was so-called depth filtered. This is to be understoodas the fact that soot particles were trapped through the whole depth ofthe filter.

[0096] Only 1 minute per element was needed to regenerate the filterunit, while consuming only 750 W to 1500 W The pressure drop over thefilter element was set to 100 mbar before regeneration.

1. An electrically regeneratable filter element, comprising at least twoflanks, each of said flanks comprising a stiff material layer, each ofsaid flanks having at least one thermally and electrically insulatedside, said filter element comprising a metal fiber fleece being pleatedaccording to pleating lines providing an edge with pleat openings,characterized in that said metal fiber fleece being mounted between saidflanks, said thermally and electrically insulated sides making contactwith said edge, said thermally and electrically insulated sides closingsaid pleat openings.
 2. An electrically regeneratable filter element asin claim 1, at least one of said flanks having an aperture, said metalfiber fleece and flanks providing a hollow filter volume, said apertureproviding entrance for gasses to inner side of said hollow filtervolume.
 3. An electrically regeneratable filter element as in claim 2,all of said flanks having an aperture.
 4. An electrically regeneratablefilter element as in claim 1 to 3, said flanks exercise a clamping forceto said metal fiber fleece in a direction essentially parallel to saidpleating lines.
 5. An electrically regeneratable filter element as inclaim 1 to 4, each of said flanks comprises a thermally and electricallyinsulating fabric and a stiff material layer, said thermally andelectrically insulating fabric being present at one side of said stiffmaterial layer, providing a thermally and electrically insulated side tosaid flank.
 6. An electrically regeneratable filter element as in claim1 to 4, each of said flanks comprises a ceramic plate, said metal fiberfleece being mounted between said ceramic plates of both flanks.
 7. Anelectrically regeneratable filter element as in claim 1 to 4, each ofsaid flanks comprising a stiff material layer, said stiff material layerand said metal fiber fleece being connected using a layer of ceramicadhesive, said layer of ceramic adhesive preventing direct contact ofsaid metal fiber fleece over the length of said edge of said metal fiberfleece with said flank.
 8. An electrically regeneratable filter elementas in claim 1 to 7, said metal fiber fleece comprising at least onemetal fiber strip, each of said metal fiber strip having two ends, acontact body being fixed to each of said ends of said metal fiberstrips.
 9. A filter unit comprising electrically regeneratable filterelements as in claim 1 to 8, said filter unit comprising a permeablecore member extending through said apertures of said flanks of saidfilter elements.
 10. A filter unit as in claim 9, said filter elementsbeing thermally insulated from each other.
 11. A filter unit as in claim9 to 10, said permeable core member being a perforated metal tube.
 12. Afilter system comprising at least one filter unit as in claim 9 to 11,said filter system comprising a valve system and an electronic controlsystem, said electronic control system controlling opening and closingof said valve system and said electronic control system controlling theprovision of electric current to said filter elements of said filterunits of said filter system.
 13. A filter system as in claim 12, saidfilter system comprising at least two filter units, said valve systemputting one filter unit off-line, said electric current being providedto at least one filter element of said filter unit which is putoff-line.